1. Field of the Invention
The present invention relates to a receiving device for receiving and demodulating GPS (Global Positioning System) waves which are spread spectrum-modulated and transmitted from GPS satellites to the earth.
2. Description of the Prior Art
The GPS satellite which was developed originally for military use has been partially opened for public use for finding the position of an object on the ground or in the ocean. A position finder which utilizes GPS waves has been used in a ship or car navigation system and in a portable position finding system, etc. The position is measured with very high precision. Recently, portable position finding systems have found many more uses and a bright future for the development of such systems is foreseen.
Electromagnetic waves transmitted from a GPS satellite include a carrier modulated with a spread-modulated signal obtained by spread-modulating a navigation message using two kinds of diffusion codes, C/A (Clear and Acquisition) code and P (precision) code which is for military purpose and is not open. There are a plurality of GPS satellites in the sky today and the C/A code and P code for each satellite is different.
A current position can be obtained by simultaneously receiving electromagnetic waves from at least three of these GPS satellites and calculating the distances from these satellites on the basis of the times of propagation for the signal from these satellites to a receiver and the propagation speed of the electromagnetic wave.
FIG. 1 shows a transmitter on a GPS satellite. The transmitter transmits a signal whose spectrum is spread by two carriers, the so-called L1 carrier having a center frequency of 1.5754 GHz and the L2 carrier having a center frequency of 1.2276 GHz.
A cesium atomic clock which vibrates very stably is used as an atomic clock 1. The atomic clock 1 oscillates at a reference frequency of 10.23 MHz and the carriers and the respective codes are produced from the reference frequency.
The L1 carrier is obtained from an L1 carrier generator 3 after multiplying the reference frequency generated by the atomic clock 1 by 154 in a 154 frequency multiplier 2 (center frequency 1.57542 GHz=10.23 MHz.times.154).
The spread code (C/A code) of 1.023 Mbps and of period of 1023 chips is generated by dividing the reference frequency by 10 in a 1/10 frequency divider 4 to generate a clock of 1.023 MHz and supplying the clock to a C/A code generator 5.
The navigation message inherent to the satellite is generated as data of 50 bps by dividing the reference frequency by 204600 in a 1/204600 frequency divider 6 and supplying it to a message generator 7.
P code is generated by a P code generator 8 which uses the reference frequency as its clock signal. Although the P code is not opened publicly as mentioned previously, it is a spread code having a code length as large as 6.187104.times.10.sup.12 chips corresponding to about one week.
The L2 carrier is obtained from an L2 carrier generator 10 after multiplying the reference frequency by 120 in a 120 frequency multiplier 9 (1.2276 GHz=10.23 MHz.times.120).
The C/A code and the navigation message thus generated are supplied to an Exclusive OR (Ex-OR) gate 11 from which the message is obtained as a spread-modulated signal. The spread-modulated signal is modulated by multiplying it with a carrier which is the L1 carrier phase-shifted by 90.degree. by means of a 90.degree. phase shifter 13 and the resultant signal is supplied to an adder circuit 18.
The P code is also supplied, together with the navigation message, to an Ex-OR gate 12 from which the message is obtained as a spread-modulated signal. This spread-modulated signal is modulated by multiplying it with the L1 carrier by a multiplier 15 and the resultant signal is supplied to the adder circuit 18.
Moreover, the P code and the respective outputs of the Ex-OR gates 11 and 12 are selected by a switch 16 which is controlled by the station on the earth. This selected signal is modulated by multiplying it with the L2 carrier by a multiplier 17 and the resultant signal is supplied to the adder circuit 18.
The respective modulated outputs are summed in the adder circuit 18 and transmitted through a transmission antenna 19 to the earth.
In a receiver on the earth, the electromagnetic wave thus transmitted from the transmitter is converted into an intermediate frequency by a frequency circuit of the receiver. It is then reverse despread-demodulated according to a C/A code output from a demodulation C/A code generator. Thereafter, the demodulated signal is further Phase-Shift Keying (PSK) demodulated to restore the navigation message.
The thus obtained navigation message is applied to a microcomputer in the receiver from which various necessary information is reproduced. The demodulation C/A generator further generates codes related to respective satellites from which navigation messages are transmitted to be received by the receiver. Switching between the satellites is controlled by the microcomputer. The most important function of the receiver is to generate C/A codes synchronized with the C/A codes of satellites transmitting messages thereto.
In a conventional receiver of this type, a synchronizing device or sync holding device is used to generate a C/A code synchronized with the C/A code of a related satellite. A delay locked loop is generally used as the sync holding device.
However, the circuit construction of the synchronizing or sync holding device of such a conventional receiver is complicated. It is also difficult to operate a delay locked loop used as the sync holding device stably with respect to the field strength of different electromagnetic waves transmitted from different satellites.
Therefore, it is usual to use a sync holding device which operates at a certain threshold value of a certain electric field strength of a received signal. However, a dynamic range of such sync holding device with respect to electric field strength of the received signal is low despite its voluminous whole circuit construction. In addition, it is difficult to receive electromagnetic waves from a plurality of satellites with correct timing, depending upon the orbital positions of the respective satellites.